Porous Storage Gives Methane a Leg Up

Hydrogen gas may be the automobile fuel of the future, but the challenges of production and storage have kept it largely off the road. Other clean fuels like methane are poised to take its place, yet current methane gas tanks come with their own storage issues. That could change thanks to the development of a new, highly porous material announced today that beats by nearly 30% the benchmark for storing methane set by the U.S. Department of Energy (DOE). What's more, because researchers can easily change the chemical makeup of the material's framework, they may be able to tailor it to store hydrogen nearly as well.

Give gasoline some credit: Burning it may spew massive amounts of carbon dioxide into the air, but the fuel is incredibly energy-rich. So any alternative needs to pack a punch. DOE has outlined a series of minimum benchmarks for how much fuel automobiles must be able to carry--enough to allow them to drive 483 kilometers (300 miles) between fill-ups. In the case of methane, that's 180 units known as v(STP)/v, or standard temperature and pressure equivalent volume of methane per volume of the absorbent material. Liquefying or storing methane under high pressure does the job. But researchers continue to hunt for materials that can store methane at low pressure, which would save energy and the cost of condensing the fuel.

Hong-Cai Zhou, a chemist at Miami University in Oxford, Ohio, hoped a new class of highly porous compounds called metal-organic frameworks (MOFs) would do the trick. MOFs are extended crystalline networks with large, open pores that make them ideal for storing gases. They're also a chemists' version of Tinkertoys, made with metal atoms at the corners and a wide variety of organic compounds that serve as the linkers in between.

Four years ago, computational chemist Randall Snurr of Northwestern University in Evanston, Illinois, and his colleagues suggested that a particular MOF structure in which the linker groups were composed of a trio of ring-shaped compounds fused together in a row would create a MOF capable of storing at least 180 v(STP)/v of methane. Zhou's team made the compound, but its methane uptake was relatively poor. A close look at the material's crystalline structure showed that part of the problem was that its pores were too small. So Zhou and his colleagues inserted a pair of additional rings, which pushed adjacent linkers farther apart from one another and expanded the pores. The upshot is that the new material, reported in today's issue of the Journal of the American Chemical Society, can store 240 v(STP)/v of methane, 28% higher than the DOE target.

"It sounds pretty exciting," says Snurr. But both he and Zhou note that MOFs are still fairly expensive to produce, so the new material isn't likely to end up under the floorboards of a car anytime soon. Also, activated carbon, a porous material derived from coal, performs nearly as well and is far cheaper. Still, because the chemical constituents of MOFs can easily be changed, there is hope that other novel compositions might perform even better, making them more economical.

Zhou says he and his colleagues are also trying to tailor their MOFs to better store hydrogen gas. That's still a big challenge: Hydrogen has so few electrons that it's hard to get it to stick to a MOF or any other material. But if MOFs can be used to beat DOE's hydrogen benchmark--something no other material has done--that could take down one of the biggest roadblocks to the hydrogen future.